Cycles Random material with alpha texture
For random colors just plug the Object Info node's random output to a Color Ramp with the desired random colors as shown bellow:



Use a constant color ramp if you want only solid colors, or any interpolated version if you want a range of variations.

For random alpha textures inside a particle system it is probably easier to just create five separate objects (they can share mesh objectdata) with five different materials applied "By object" with your different alpha maps.
Group these objects into a single group with Ctrl+G and the use that group as dupli in the particle system.



Turn on the "Pick Random" option and the particle system will take care of choosing each object arbitrarily.